In order to suppress electromagnetic interference (EMI), in most cases an input filter is required on the input side of switched mode power supplies (SMPS). In addition to an inductance, EMI filters typically include one or more capacitors connected between the mains terminals. These capacitors are also known as XCaps. EMI filters typically also include one or more capacitances connected between one of the mains terminals and a protective Earth (which are also known as YCaps). Typically, this protective Earth takes the form of a secondary ground, to which the Y-Caps are connected, whereas the bridge-rectifier is grounded to a separate primary ground; the primary and secondary grounds have mains separation, but may typically be connected by one or more further YCaps.
It is desirable, and in some regulatory regimes it may be mandatory, that the voltage between the mains terminals of the switched mode power supply is reduced to a safe value within a certain time after the mains is unplugged. Otherwise, there remains a risk of electrical shock to the user, by inadvertently contacting the terminals of the plug.
As examples, according to the international Safety Requirement standard, IEC600665 for audio, video and similar electronic apparatus, it is required that within 2 seconds of a power supply being unplugged, the voltage between the terminals should be less than 60 V. Further, according to IEC60950, the decrease should occur within one second.
When the mains supply to the SMPS is disconnected, for instance by the mains being unplugged, residual charge on the XCaps and YCaps may initially result in a voltage which is above the safe level.
If the power supply is operating and fully loaded, the load will in most instances rapidly result in the discharge of the charge on the capacitors. However, under no-load conditions, the power supply may have been disabled; also, under conditions where the mains voltage has fallen below acceptable levels, (so-called “brownout” conditions), the power supply may have been switched off. Under such conditions, capacitances on the output side of the rectifier may prevent discharge of the XCap and YCap capacitors, on the input side of the rectifier, to an acceptable level within an acceptable period.
Thus, further measures are required to ensure the capacitors are appropriately discharged under such conditions. It is known to provide an additional resistor discharge network in parallel with the mains input, in order to allow for such discharging. In one known configuration, the discharge resistors are adapted to also be used as sense resisters, to measure the instantaneous mains voltage, for brownout protection.
Existing resistive discharge solutions suffer the disadvantage that the resistors dissipate power. Although this power dissipation, which for a typical application may be between 20 and 100 mW, is insignificant under most fully loaded conditions, under conditions of no-load, it is relatively appreciable.
It would be desirable to provide a method of discharging XCaps and YCaps which does not results in this continuous power drain.